Progress towards an innovative ion energy analyzer for electric potential diagnosis of fusion plasmas

Knowledge of the electric potential as a function of location and time in magnetically confined plasmas can advance understanding of the electric field and its effect on transport. A diagnostic heavy particle beam can be used to obtain such measurements, as well as fluctuations of potential and density in the interior of hot plasmas. To determine the electric potential within the plasma accurately, the difference in energy of the injected particles and detected ions must be precisely measured. Ion energy analyzers commonly achieve this by deflecting the ions with an electric field provided by a pair of flat, parallel electrodes. We are developing an innovative ion energy analyzer that employs cylindrical annulus sector electrodes rather than flat electrodes. It simultaneously measures the incoming angle of the beam and the deflection by the electrodes in order to determine the ion energy. This novel combination is key to obtaining equilibrium plasma potential measurement resolution of .01% of the beam energy. This design makes more efficient use of the electric field region (than traditional flat parallel plates having a single bias-voltage), reduces the maximum electrode voltage required, and lowers overall cost. Important features of the analyzer's design and characterization and initial measurements made with the analyzer will be discussed. Portions of this work were done at the Wisconsin Plasma Physics Laboratory (DoE award number DE-SC0018266) and on the Madison Symmetric Torus.